Electrical resistor and method of making same



Aug. 24, 1965 B. F. HAY 3,201,855

ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Filed Feb. 21, 1961 IN V EN TOR.

BRA/[7P0 5 A67 1577/1 52? #019440 6, Tear/.95

United States Patent 3,201,855 ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Bernard F. Hay, Columbus, Nebn, assignor to Dale Electronics, Incorporated, Columbus, Nebr- Filed Feb. 21, 1961, Ser. No. 90,794 1 Claim. (Cl. 29-155.63)

This invention relates to electronic units that objectionably overheat, and more particularly to electrical resistors.

Electrical resistors are now being used in many types of electronic equipment, including aircraft of various types.

The most serious problem experienced in the use of resistors is their tendency to objectionably develop very high temperatures under electrical loads. Some effort has been made to reduce this heat factor by placing the resistor inside a metal housing and then securing the housing to a metal object whereby some heat could be transferred away from the resistor. Other objections to resistors are that they are relatively heavy, are poorly protected against shock and are difficult to manufacture if they are to be properly manufactured for high eificiency.

Therefore, one of the principal objects of my invention is to provide an electrical resistor unit of high efficiency that is easily'and quickly manufactured in volume.

A further object of this invention is to provide an electrical resistor unit that is of light weight relative to its high voltage capacity.

A still further object of this invention is to provide an electrical resistor that is highly efiicient in the dissipation of heat.

A still further object of this invention is to provide an electrical resistor unit that is successfully protected against severe shocks.

Still further objects of my invention are to provide an electrical resistor unit that is economical in manufacture, durable in use and refined in appearance.

These and other objects will be apparent to those skilled in the art.

My invention consists in the construction, arrangements, and combination, of the various parts of the device, whereby the objects contemplated are attained as hereinafter more fully set forth, specifically pointed out in my claim, and illustrated in the accompanying draw ings, in which:

FIG. 1 is a perspective view of my resistor unit ready for use;

FIG. 2 is a side sectional view illustrating one method of introducing under pressure a solidifiable insulating and holding plastic material into the outer metal housing;

FIG. 3 is an end view of the injection plastic molding fixture taken from line 3-3 of FIG. 2, and

FIG. 4 is an end view of my resistor unit installed onto a supporting base.

As herebefore indicated, the preferred resistor is one that is light of weight, compact, and one that will Withstand severe impacts, large electrical loads and high temperatures without fracture or malfunction. While this invention may be employed for other electrically heatable units, I am particularly concerned with the proper and desirable fabrication of electrical resistors. The physical features of ordinary electrical resistors consist of a cylindrical body having therein the usual wire coil, and caps 11 and 1 2 at each end, respectively, of the body 10. Such resistors have the usual two lead conductors 15 and 16 extending outwardly through the two vertical end caps 11 and 12, respectively, as shown in FIG. 2.

. Such a resistor is limited in the load it will carry depending on its size, and in order to accommodate the maximum loads required, it is necessary to employ much 3,291,855 Patented Aug. 24, 1965 larger resistors than desirable in order to prevent excessive heating, fracture, or other damage. It is essential and important to provide for greater heat-dissipation than heretofore possible in order to employ a resistor of small size while carrying maximum electrical loads.

To produce a desirable resistor unit, my first step is to place, loosely around, the cylinder portion or body 10, an aluminum hollow cylinder 17. This cylinder is open at both ends at this stage of assembly and the two leads 15 and 16 extend through these two open ends respectively. One side of the hollow cylinder is flat to form a base portion 19 for engagement with a supporting base, frame, chassis or like. The cylinder base portion has holed flanges 2t) and the cylinder may be rigidly secured to a supporting base 21 by any suitable means such as screws 22 or bolt means extending through the hole of the flanges 20. Cooling ribs or flanges 23 may be formed on the outer periphery of the hollow cylinder 17. The bore 24 of the hollow cylinder is substantially greater than that of the diameter of the inserted resistor. Also, it will be appreciated that the bore of the hollow cylinder is of sufiicient length to more than embrace the body 10 and caps 11 and 12 of the resistor. After the resistor is properly spaced and positioned within the bore 24, my next step is to fill all spaces inside the bore and around the resistor by suitable castable insulation material. Obviously, for the operation to be successful, the castable material should be introduced into the bore under pressure. A congealing liquid plastic is not recommended. I recommend instead a transfer molding process which uses a powder or pellet type insulation which when introduced under pressure and heat first liquefies and then hardens into a very dense solid plastic insulation. By the use of the transfer molding process as compared to the liquid plastic, I have raised the operation of the resistor unit from approximately eighteen watts to twenty- =five watts. This increased effectiveness is at least partly due to the increased density of the hardened plastic insulation which is more efficient in transferring heat from the resistor directly to the metallic hollow cylinder 17, and which in turn dissipates heat to the atmosphre and the base support to which the unit is secured. Also, by the insulation being pressure molded in the bore, the resistor therein will not only be completely imbedded (except its leads) in the plastic insulation but will be supported properly in all direcitons and rigidly held against detachment from the housing 17. There are high-temperature plastic insulation materials which are suitable and I recommend materials in the classifications of epoxys, phenolics or si-licones. These three compounds with perhaps suitable mineral fillers are excellent for transfer molding. The use of powder or pellet material plastic casting is usually known as pressure casting or plastic injection molding. Pressure plastic molding machines use both heat and pressure to solidify the powder or pellet plastic material within the cavity mold.

In the cast assembling of my unit, its own housing 17 acts as the mold cavity except for the two ends of such a mold cavity. Any suitable injection molding machine may be used. In the drawings I use the numeral 25 to designate the :base of a molding machine. The numeral 26 designates a rigid block on the base 25 and which has a vertical inner bearing surface (FIG. 2) and which acts as the one end of the mold cavity. The numeral 27 designates the slidable gate of the molding machine and which has a vertical inner bearing surface (FIG. 2) and which acts as the other end of the mold cavity as shown in FIG. 2. The nncompleted resistor unit is placed between the two members 26 and 27, and the number 27 slid into closed position. The two conductor leads \15 l and 16 must extend through the two members 26 and 27 respectively, and this may be accomplished by having the two members 26 and 27 'detachably'split as shown in FIG. 3. The vertical beaning surface of end member block 26 engages the'verticalleft end of the housing 17 and the vertical bearing surface ofgate block member 27 engages the vertical right end of. the housing 17, 7 By this arrangement of-parts; the resistor housing 1-7 and members 26 and 27'forim the mold cavity. The plastic material 29 is then injected-[into the mold cavity-under pressure through the conduit 30 and which communicates with the inside of the mold cavity through the end member '26. As indicated in FIG; 2, the gate 27 is moved to the left into tight binding abutting engagement only with-the righthand vertical end o-f the housing so that the gate 27'and wall 26 can effectively seal the, interior of the bore 29 to prevent the escape of the plastic material 29 therefrom whichis being introduced into,

the bore under normally high pressures'from the injection molding machine being used.

maohinejis. opened and :the entire resistor unitfsuch as shown'in. FIG. 1, is removed therefrom, Asshown in FIG. 1, no surplus plastic remai-ns'either on the lead 16 a or on the exposed, vertical end surface of the housing 17,, which means that the fabrication: of the unit is finished; I

as soon as it is taken from the molding machine.

- Some changes may be made in the construction and arrangement of 'my' electr-ical' resistor and method of" making same without departing from the real spirit and purposeof my invention, and it is my intention to cover.

by my claim, any modified forms of structure or use of mechanical equivalents which-may bereasonably included within its scope. a

Iclaim: I i

- The method of producing an electrical resistor comprised of a housing having opposite vertical ends and,

an elongated horizontal bore extending 'therethrough from one end to the other, and. a resistorelement im bedded within a mass'of hard insulative material within said bore, wherein said resistor element has an elongated body with axially-positioned leads extending from opposite ends thereof, and said elongated body is shorter than said bore except for said axial leads which have lengths extending beyond the lengths of said housing,

said elongated body having a proportionately smaller cross-sectional area than said'bore, comprising, 7

placing said resistor element in the bore of said'housing whereby a space is created entirely" around the elongated bodyof said resistor element,

passing the ends of said leadsthrough separate closure.

means having opposite fvertical bearing surfaces so that said resistor element is supported only by said leald's, V v I r placing s aid clos ure mean-s in engagement with said housing and imposing diametrically opposite forces on said closure means so that the, vertical bearing surfaces of said closure means are in tight, binding, abutting and sealing contact only with the vertical ends of said housing whereby said housing is tightly grippedand held against longitudinal displacement-by said closure means, V g 1 i providing an access opening into said bore,

introducing .a single-harde'nable insnlative fluid from the high pressure system of'an injection molding machine through said access opening tofill the space in said bore around the entire'outer surface of said elongated body of said resistor element, allowing said hardenable insulative fluid to harden to permanently imbe-d said resistor element in "said bore of saidhousing, I and removingsaidQclosuremeans from engagement 4 with said housing and from supporting engagement with the leads of said resistor element. 7 v

'R'eferen'cesCitedby the Examiner UNITED STATES PATENTS 2,176,604v 10/39 'Benkelrnan 'I- 29-15569 2,478,754 '8/49 Dnblier etal. 53-36 ,2',535,808 =12/50 M-ucher. V 2,558,798 7/51 Thom, 338-256 2,577,005 -1'2/51 Di Giacomo 264-27-2 2,629,166; 2/53 Marsten et' al. 29--155.5 2,693,023 11/54 Kerridge 29,l55.5 2,749,456 a 6/56 Luenberger. I 2,857,498 10/58: Risk 338- 256 2,904,618 9/ 59 Robinson et a1. 7 r 2,943,359 7/6 0 Sussman c 29-15563 3,044,127 7/62; Alden. r a i 5 3,048,914. 8/62 Kohring 29 155.63

, Q FOREIGN PATENTS: 824,828 12/59; Great Britain.

WHITMORE A. WVIYLTZ, Primary Examiner, RAY K; WINDHAM, JOHN F. CAMPBELL,

Examiners. 

